1. Field of the Invention
The present invention relates to rotary-type drill bits for use in subterranean drilling. Particularly, the present invention relates to rotary-type earth-boring drill bits that employ hydraulics to at least partially achieve or tailor a desired lateral force balance or net force imbalance characteristic or moment configuration.
2. Background of Related Art
Fixed cutter rotary drag bits for subterranean earth-boring have been employed for decades. It has been found that increasing the rotational speed of such a drill bit, for a given weight on bit, increases the rate of penetration of the drill bit. Increasing the rotational speed of a drill bit, however, may tend to prematurely damage or destroy the cutting elements of the drill bit, which decreases the useful life thereof.
It has been recognized that cutting element destruction, particularly at higher drill bit rotational speeds, is at least in part attributable to a phenomenon known as "whirl" or "bit whirl". Radially directed centrifugal imbalance forces (i.e. transverse to the axis of the drill bit) exist to some extent in every rotating drill bit and drill string. Such forces are attributable in part to mass imbalance and in part to dynamic forces generated by contact of the drill bit with the formation. In the latter instance, aggressive cutting element placement and orientation creates a high tangential cutting force relative to the ordinary force of a rotating bit and aggravates the imbalance. In any event, these imbalance forces tend to cause the drill bit to rotate or "roll" about the borehole in a direction counter to the normal direction of rotation imparted to the bit during drilling. This counter-rotation is termed "whirl", and is a self-propagating phenomenon, as the side or transverse forces on the bit cause its center of rotation to shift to one side, after which there is an immediate tendency to shift again. Since cutting elements are designed to cut and to resist impact received in the ordinary direction of bit rotation (clockwise, looking down), whirl-induced contact of the cutting elements with the borehole wall in a counter-clockwise direction places stresses on the cutting elements for which they were never designed, and may damage or destroy the cutting elements, which decreases the effective life of the drill bit.
Balancing the transverse forces acting on rotary-type drill bits during operation has proven to be extremely important to their performance. Drilling forces influence dynamic behavior such as whirl, steerability, and bit walk. Both balanced force and net force imbalance drill bits have been developed to counteract the tendency of a drill bit to whirl.
In many conventional balanced rotary-type earth-boring drill bits, the various components, such as the blades and cutting elements, are designed and oriented in a manner which balances the forces acting on the drill bit and reduces the tendency of the drill bit to whirl when rotated at high speeds and under a low weight on bit (WOB).
Rotary-type earth-boring drill bits which have a net transverse imbalance force during the rotation thereof have also been designed in order to reduce or eliminate bit whirl. Such drill bits are designed to focus or direct the imbalance forces, which typically cause bit whirl, to a particular side of the bit. Typically, the cutting elements and mass of the bit are positioned in a manner that causes the bit to ride on a low friction bearing zone or pad on the gage of the side of the bit to which the imbalance forces are focused, thus substantially reducing the drill bit/borehole wall tangential forces which typically induce whirl. This solution is disclosed in U.S. Pat. Nos. 4,982,802, 5,010,789 and 5,042,596, each of which are assigned to Amoco Corporation of Chicago, Ill. The magnitude of net transverse imbalance forces that are focused toward the low friction bearing zone may, however, be more than adequate to maintain contact of the low friction bearing zone of the drill bit with the formation. Consequently, an excessive net imbalance force may cause the low friction bearing zone to wear prematurely and therefore shorten the useful life of the drill bit.
While the forces of the drill bit against a rock formation and the consequent effect of such forces on the drill bit, as well as the positioning of various components on the drill bit, are typically considered during the design of both balanced and net imbalance rotary-type earth-boring drill bits, the designs of conventional rotary-type drill bits do not account for the hydraulic force of drilling fluid that flows through and out of the drill bit.
Both balanced and net imbalance drill bits include nozzles through which drilling fluid, such as "mud", air, gases or gas mixtures, foam, or other fluid media that may be employed during drilling of a formation, flows. Drilling fluid is typically used during drilling in order to cool and lubricate the face of the drill bit and the cutting elements thereon, to remove debris from the bit face and transport same up the borehole, and to prevent the introduction of pressurized gases or liquids from the drilled formation into the borehole, which may otherwise result in a "blowout" of the borehole.
As drilling fluid flows through the drill bit the hydraulic forces of the drilling fluid affect the rotational balance of the drill bit. The hydraulic forces of drilling fluid on a rotary type drill bit are similar to the force that occurs as water flows through and out of a garden hose or fire hose. In rotary-type drill bits, hydraulic forces will affect the rotational balance of the bit as the drilling fluid moves through the bit, accelerates, and exits the bit through the nozzles. Typically, the nozzles of a drill bit are fixed in orientation; thus, drilling fluid exits the bit through each nozzle in a fixed direction.
Hydraulic forces can have a substantial effect on the balance or net imbalance of a rotary-type drill bit due to the typically high density of drilling fluid and the typically high flow rates of drilling fluid through the bit during drilling. The line graphs of FIGS. 1 and 2 illustrate the amount of hydraulic force that is exerted on a drill bit as drilling fluid having a weight of 12 lbs/gallon flows at a given rate (typically measured in gallons per minute, or "GPM") through variously sized nozzles of the drill bit. For example, about 20,000 net pounds of force is exerted on a drill bit having four 8/32 inch nozzles as 12 lbs/gallon mud flows therethrough at a rate of 320 GPM. As another example, about 5,000 pounds of force would be exerted on a drill bit including four 15/32 inch nozzles as 12 lbs/gallon mud flows therethrough at a rate of 320 GPM. Thus, since drilling fluid typically exits the drill bit under high flow rates and in a fixed direction, the balance of the drill bit may be undesirably affected, which may induce whirl in balanced force or net imbalance force drill bits which are otherwise designed to prevent this phenomenon, or alter the amount or direction of net imbalance of a net imbalance force drill bit so as to orient the force in a direction so that the bit does not ride on its bearing surface or pad.
Accordingly, a rotary-type drill bit is needed in which hydraulic forces may be utilized to provide a desired amount of force balance or net force imbalance characteristic of the drill bit. A system for altering or tailoring the overall balance of a rotary-type drill bit is also needed.